As energy crisis and environmental pollution problem are increasingly prominent, the development of a renewable, environmentally-friendly and alternative energy source with high specific energy is very urgent.
Lithium ion batteries with advantages of providing high specific energy, high conversion efficiency, long cycle life and low self-discharge rate and being environmentally-friendly, and the like, have been considered to be an ideal green energy and have wide application. However, now, lithium ion batteries are mainly used in electronics industries, such as communication and IT, in which small batteries with a capacity of less than 5 AH (ampere hours) are mainly involved. With recent increases of urban environment pollution and the popularization of the usage of wind and light energy, demand for large power source and energy storage system, such as various electric motorcycle, electric car, electric bus, UPS, and energy storage cell stack using wind or solar power, is rapidly increasing. Theoretically, small batteries can be connected in series (in parallel) or in parallel-series to form a large power source so as to meet the electricity demand of large electrical equipments.
However, in practice, current imbalance is apt to occur at charge and discharge ends of the batteries in series, because the batteries in series call for consistency of each single battery, and any one of them going wrong should affect the performance of the entire battery stack. Further, the battery management system is necessary for the use of the lithium ion batteries. However, the current battery management system is effective for batteries in series only, and there is no any electronic technique for the management of the batteries in parallel and also a lack of studies on method and index of evaluation for the performance of the batteries in parallel. Therefore, it is hard for small batteries in parallel to meet needs for a large-capacity battery from technical and industrial points of view.